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DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON. Director 




No. 347 
HEAT TREATMENT OF DURALUMIN 

BY 

P. D. MERICA, Physicist 
R. G. WALTENBERG, Assistant Physicist 
H. SCOTT, Assistant Physicist 
Bureau of Standards 



ISSUED NOVEMBER 15, 1919 





PRICE, 10 CENTS 

Sold only by the SupcriDtendcnt of Doaiments, Government Printing Office 
Washinston, D. C. 




WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1919 



DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON, Director 



No. 347 
HEAT TREATMENT OF DURALUMIN 

BY 

P. D. MERICA, Physicist 
R. G. WALTENBERG, Assistant Physicist 
H. SCOTT, Assistant Physicist 
Bureau of Standards 



ISSUED NOVEMBER 15, 1919 




PRICE, 10 CENTS 

Sold only by the Superintendent of Documents, Government Printine Oflice 
Washington, D. C. 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1919 



if^€-^ 



1 






^ 






HEAT TREATMENT OF DURALUMIN 



By P. D. Merica, R. G. Wallenberg, and H. Scott 



CONTENTS Page 

I. Introduction 271 

II. Composition and preparation of alloys 273 

III. Heat treatment and aging 273 

1. Effect of quenching temperature 278 

2 . Effect of aging temperature 281 

3. Effect of temperature of quenching bath 285 

4. Effect of prior heating at quenching temperature 293 

5. Effect of preheating to 515° C before quenching from lower tem- 

peratures 293 

rV. Miscellaneous tests 293 

1. Density and dilatation 296 

2. Electrical resistivity 297 

V. Mechanism of hardening during aging after rapid cooling 299 

1. Structure of duralumin 306 

2. Analogy between the hardening of duralumin and that of steel ... 310 

3. Eutectic structure and influence of magnesium 311 

VI. Conclusions relative to the manufacture and heat treatment of duralumin . 3 14 

VII. Summary and conclusions 315 

I. INTRODUCTION 

The remarkable phenomena exhibited by the aluminum alloy 
known as duralumin were discovered during the years 1 903-191 1 
by A. Wilm^' ^ and have been described by him and others. ^' *• ^' * 

The unusual feature of this alloy is the fact, as was shown by 
Wilm, that it can be hardened quite appreciably by quenching 
from temperatures below its melting point followed by aging at 
ordinary temperatures, which consists merely of allowing the 
material to stand at these temperatures. The hardness is not 
produced by the quenching alone, but increases during the period 

• A. Wilm, Physical-MetallurKical investiaations of aluminum alloys containing maijnesium, Metallurgic 
8, p. 325; 1911. 

' A. Wilm, The liarclcninR of li;:ht aluminum alloys, MetallurRie, 8, p. 650. 

• L. M. Cohn, Duralumin, Vcrh. Z. Bcforderinu dcs Gcwerbcflcisscs, 89, p. 543; 1910. 

• L. M. Cohn, Changes in the jiliysical proiwrties of aluminiun and its alloys, with special reference to 
duralumin, Elektrotechnik u. Ma«'Iiincnbnu, 31, p. 430; 1913. 

• L. M. Cohn, Duralumin. Ivlt-ktrolechnik u. Mascliinenbau, 30, pp. 809, 829; 1912. 

' P. D. Merica, Aluminum and its litht alloys, Cirailar 76 of the Bureau of Standards, 191.S, also Chcm. 
and Met. Enjt., 19, pp. 135, 200, 329, 587, 63s, 729, 780; 1918. 

271 



272 Scientific Papers of the Bureau of Standards {Voi. is 

of aging, which may be from one to three days. Cohn (see notes 
3 and 5, p. 271) gives data showing the increase of hardness of 
duralumin during aging, after quenching in water from about 
450° C. Upon annealing the alloy so hardened by aging, it is 
softened exactly as is hardened steel. 

The composition of this alloy usually varies within the following 
limits : 

Per cent 

Copper 3 -4. 5 

Magnesium o. 4— i. o 

Manganese o — o. 7 

Aliiminum Balance 

Iron (as impurities) o. 4— i 

Silicon (as impurities) o. 3 — o. 6 

Its density is about 2.8. It is used only in the forged or rolled 
condition. 

This alloy has been produced for some years commercially and 
is in demand for the fabrication of parts for which both lightness 
and strength are required, such as for aircraft. Its tensile strength 
will average 50 000 to 60 000 poimds per square inch after appro- 
priate heat treatment, such as that described by Wilm. 

With the purpose of ascertaining whether the heat treatment 
described by him actually developed the best mechanical proper- 
ties possible for diu-alumin, the authors undertook a study of the 
effect of variation in heat-treatment conditions, that is, quenching 
temperature, aging temperattire, etc., upon these properties and, 
in connection with another investigation,^ a study of the effect of 
chemical composition upon them. 

E- Blough had already called the attention of one of the authors 
to the fact that the amount of hardening produced by heat treat- 
ment was influenced quite markedly by the temperature from 
which the material was quenched, a most interesting fact which 
was not brought out by Wilm's published investigations, which 
mentioned merely the effect of aging after quenching from one 
temperature, in the neighborhood of 450° C. 

An explanation was sought also for the mechanism of hardening 
during aging of this alloy, and additional data were obtained 
bearing upon this phase of the matter. 

The experiments here described were carried out partly in the 
laboratories of the Btureau of Standards and partly in cooperation 

' p. D. Merica, R. G. Waltenberg, and A. N. Finn. The tensile properties and resistance to corrosion of 
rolled light alloys of aluminum and magnesium with copper, with nickel, and with manganese. Techno- 
logic Paper No. 132 of the Bureau of Standards, 1919. 



I 



M erica, Wallenberg, '\ 
Scott J 



Heat Treatment of Duralumin 



273 



with the Aluminum Company of America in its laboratories at 
New Kensington. The alloys used were prepared at the New 
Kensington plant of this company, and the authors wish to express 
their appreciation of the assistance and cooperation which has 
been given throughout by this company through E. B lough, 
chief chemist. H. H. Beatty of Mr. Blough's staff was active in 
assisting this work. 

11. COMPOSITION AND PREPARATION OF ALLOYS 

In Table i are given the chemical compositions of the alloys of 
the aluminum-copper-magnesium series which were used in these 
experiments. The ingots, 1 2 by 24 by 3 ^ inches, were rolled hot 
at about 410° C to ^ inch thick and thereupon cold rolled to 
0.081 inch (12 B. & S. gage), annealed at about 425° C, rolled 
cold to 0.051 inch (16 B. & S. gage), annealed again and cold 
rolled to 0.032 inch (20 B. & S. gage). The casting and rolHng 
was done at the New Kensington plant of the United States 
Aluminum Co. 

TABLE 1. — Chemical Composition of Alloys « 



Number 



Aluminum 


Magnesium 


Copper 


Iron 


97.27 


1.16 


0.72 


0.56 


96.69 


2.37 


.04 


.62 


97.15 


None 


2.15 


.36 


96.65 


2.84 


.04 


.27 


96.11 


None 


3.19 


.40 


96.72 


2.03 


.72 


.30 


96.62 


1.00 


1.80 


.35 


96.68 


1.07 


1.67 


.33 


95.98 


3.50 


.08 


.26 


95.83 


2.95 


.74 


.27 


95.51 


1.26 


2.58 


.41 


95.74 


.46 


3.18 


.34 


95.48 


.64 


3.22 


.39 


96.80 


1.06 


1.56 


.32 


94.36 


1.08 


3.74 


.52 


94.47. 


1.06 


3.68 


.50 



Silicon 



CI.... 
C2.... 
C3... 
C4... 
C5... 
C6.... 
C7.... 
C8... 
C9... 
CIO.. 

oil... 
C12... 
Ai-12 

E3... 
N34.. 
E4... 



0.29 
.28 
.34 
.20 
.30 
.23 
.23 
.23 
.18 
.21 
.22 
.24 
.27 
.26 
.30 
.29 



o Aluminum by difference. 



III. HEAT TREATMENT AND AGING 

Tensile tests and scleroscope measurements were made upon 
specimens taken (i) from the sheets as rolled, (2) from the rolled 
sheets, annealed, and (3) from the rolled sheets after heat treat- 
ment consisting of heating to various temperatures in a gas or 



2 74 



Scientific Papers of the Bureau of Standards 



[Vol. IS 



electric furnace, quenching in water, and aging at room or other 
temperatures for different periods. The results of these tests are 
ffiven in Table 2. 



TABLE 2. — ^The Tensile Properties and Scleroscope Hardness of Rolled, of Annealed, 
and of Heat-treated Aluminum-Copper-Magnesium Alloys 





As rolled 


After annealing at 422° C 


. Number 


Sclero- 
scope 
hardness 
magni- 
fying 
hammet 


Tensile 

strength 


Elonga- 
tion in 
2 inches 


Sclero- 
scope 
hardness 
magni- 
fying 
hammer 


Tensile 
strength 


Elonga- 
tion in 
2 inches 




42 

19 

35 

37 

34 

38 

44 
38 

38 

45 

50 
31 


Lbs./hi.2 

49 000 
48 400 

48 600 

49 600 
25 800 
23 600 
23 600 

34 900 

35 700 

34 000 
38 400 
38 600 
37 200 

35 900 
37 500 

37 700 
35 300 

38 500 
38 100 

44 200 

45 500 
45 300 
38 100 
38 100 
41 200 

43 200 
41 200 

44 800 
44 600 
47 500 
56 700 
52 900 
58 400 
38 900 
38 600 


Per cent 
2.0 
2.5 
2.5 
2.5 
4.0 
3.0 
3.5 
2.5 


15.5 

7.5 
7.0 

10.5 
8.0 

13.0 

17.0 
12.5 

12.0 

12.0 

15.5 
7.5 


Lbs./in.2 
33 000 
33 100 
32 700 


Per cent 


CI 


15.0 




14.0 


C2 


16 600 

15 900 

16 100 
21 600 

21 800 

22 000 
29 200 
29 200 

29 400 

23 000 
22 400 
22 800 

30 500 

29 900 

30 800 

35 300 
34 600 
34 800 

28 500 

29 100 

31 600 
31 200 

30 500 
30 600 
30 200 
30 200 
34 900 

36 000 




35.0 
35.0 
33.0 
31.0 
33 


03 




1.5 


33.5 
18.0 


C4 


1.5 
l.S 


18.0 
21.0 
30.0 




C5 


2.5 
2.0 
1.0 
0.5 
l.S 
2.0 
2.0 


28.5 
32.5 




C6 .. 


18.5 

,16.0 

26.0 

25.5 
25.0 


C7 




C8 


1.5 


18.5 




18.5 


C9 


1.5 
1.5 
1.5 
1.5 
1.5 
1.5 
2.0 
1.5 
2.0 
5.0 
5.0 


17.5 
17.5 






CIO . 


19.0 
19.0 


Cll 


17.0 
20.5 
24.0 






C12 


23 100 
23 000 


24.0 




24.0 



i 



■Mta 



Merica, Wtdtenberg.l 
Scoa J 



Heat Treatment of Duralumin 



275 



TABLE 2. — The Tensile Properties and Scleroscope Hardness of Rolled, of Annealed, 
and of Heat-treated Aluminnm-Copper-Magnesium Alloys — Continued 



Number 



After heat treatment consisting of quenching in water and aging 



Quenched from 478° C 



Aging 



CI 



<2 



C3. 



Cf 



C5 



C6 



CI 



C8 



C9 



CIO 



CU 



CU 



Aged 

at 

110° 



Days 



Aged 
at 
20° 



Days 

11 
11 
11 



Scleroscope 
hardness 

magnifying 
hammer 



20 



23.5 

26 
28.5 



28 
31 



Tensile 
strength 



Lbs./in.« 

36 870 

37 080 
36 260 



16 830 
16 510 
16 510 



28 020 
25 810 
25 440 



29 300 
28 910 

30 280 



33 220 
32 580 
31 930 



31 050 
33 790 
31 640 



42 350 
42 530 
42 350 



46 400 

47 030 

48 900 
47 650 
31 790 
30 450 
30 070 



38 030 

37 630 

38 430 



50 450 
48 950 

51 740 
50 880 
38 330 
38 730 
35 910 



Elonga 
tion in 

2 
inches 



P.ct. 

18.5 
17.0 
14.0 



39.0 
38.5 
38.5 



15.5 
25.0 
25.0 



20.0 
20.0 
22.0 



15.0 
18.5 
16.0 



20.0 
19.0 
17.5 



21.0 
21.0 
21.0 



20.0 
22.0 
21.5 
18.0 
14.0 



26.0 
25.0 
22.5 



22.0 
22.0 
22.0 
14.0 
13.5 
12.5 



Quenched from 510° C 



Aging 



Aged 
at 

110° 



Days 



Aged 
at 

20° 



Days 



Scleroscope 
hardness 

magnifying 
hammer 



17 
27 



24 
35 

22 

32 

13 

14 

14 

26 

29.5 

34 

25-28 

26 



Tensile 
strength 



Lbs./in.2 

38 030 
37 220 

48 120 
47 210 
16 670 
16 670 
16 510 
16 510 

26 350 

27 690 

29 420 
27 790 

30 060 

29 700 

31 590 
31 350 
31 960 

30 500 
30 910 
33 970 
33 370 
33 950 
43 190 

43 560 
45 650 
45 740 

53 970 
52 250 

44 130 
44 910 

49 680 
51 530 
29 120 

29 500 

30 270 
30 270 
37 430 
37 630 
47 690 
47 690 
51 520 

50 870 

54 740 

55 590 
42 370 

39 340 
49 230 
49 830 



Elonga- 
tion in 

2 
inches 



P. ct. 
17.0 
16.5 
16.0 
18.5 
34.0 
33.0 
28.0 
33.0 
19.0 
11.5 
20.0 
19.5 
23.0 
16.5 
19.0 
20.0 
15.5 
14.0 
19.0 



17.0 
23.5 
18.5 
18.0 
18.5 
19.5 
20.0 



24. S 
23.0 
19. S 
17.0 
21.0 
22.0 
23.0 
22.0 

24. S 
21. S 
21.5 
22.5 
21.0 
24.0 
23.0 
20.0 
14.5 
16.5 
26.5 

25. S 



276 



Scientific Papers of the Bureau of Standards 



\Vol. IS 



TABLE 2. — The Tensile Properties and Scleroscope Hardness of Rolled, of Annealed, 
and of Heat-treated Aluminum-Copper-Magnesium Alloys — Continued 







After heat treatment consisting of 


quenching in water and aging — Continued 






Quenched from 520° C 


Quenched from 525° C 


Number 


Aged 
8120° 


Scleroscope 
hardness 

magnifying 
hammer 


Tensile 
strength 


Elonga- 
tion in 

2 
inches 


Aging 


Scleroscope 
hardness 

magnifying 
hammer 


Tensile 
strength 


Elonga- 




Aged 

at 

110° 


Aged 
at 
20° 


tion in 

2 
inches 


Cl 


Days 

11 
11 

11 
11 

11 
11 

11 
11 

11 
11 

11 
11 

11 

11 

11 
11 
11 
11 
11 
11 

11 
11 


18.5 
8 
13 
12 
14 
17 
25 
25.5 
13 
14.5 


Lbs./in.> 

35 870 

36 910 


P.ct, 

18.5 
19.0 


Days 

3 
3 

3 
3 

3 
3 

3 
3 

3 
3 

3 
3 

3 
3 

3 
3 

3 
3 

3 
3 


Days 

11 
11 
8 
8 
11 
11 
8 
8 
11 
11 
8 '' 

8 

1 
11 

11 

8 

8 

11 

11 

8 

8 

11 

11 

8 

8 

11 

8 
8 
U 
U 
8 
8 
11 
11 
8 
8 
11 
11 
8 
8 


i 
16 

27 

8 

13 

10.5 

12 

17 

10.5 

12 

15 

26 

16 

35 

25 

31 

13 

18 
15 
26 


Lb8./in.» 

37 220 

38 130 
50 140 
49 930 
16 350 
16 870 

16 980 

17 340 
28 550 

28 640 
26 800 
26 980 

29 600 
29 100 

34 850 

35 350 
29 540 

32 490 
28 170 

1 27 960 

34 980 

35 470 
47 580 

45 840 

46 850 
46 760 
54 170 
56 110 
46 520 
46 030 
53 440 
53 000 

33 790 
32 510 

34 800 

34 890 

35 160 
35 430 
44 260 
46 450 


P.ct. 
14.0 
17.0 




17.0 








17.5 


C2 


13 970 

14 290 


37.0 
35.0 


35.0 




31.0 








30.0 


C3 


24 700 

25 440 


18.5 
21.0 


23.0 




17.5 








20.0 


C4... . 


27 540 
27 150 


22.0 
20.0 


20.5 
22.0 




17.0 








16.5 


C5 


29 150 
29 580 


13.0 
16.0 


19.0 
16.5 




23.0 








26.0 


C6 


36 140 
33 990 


21.5 
19.5 


17.5 
18.0 




20.5 








16.5 


C7 


42 530 
42 160 


22.5 
22.5 


20.5 
19.5 




21.0 








19.0 


C8 


45 160 
44 720 
44 720 
44 070 

29 500 

30 070 


26.0 
25.0 
21.0 
22.0 
22.5 
19.5 


23.0 
25.5 


C9 


21.0 
19.0 
21.0 
17.5 




17.5 








18.0 


CIO.. 


37 430 

38 230 


23.0 
22.0 


22.5 
22.0 




19.9 








20.0 


cu 








C12 










I 3 


11 
11 
8 


1 " 

26 


1 42 660 
1 36 500 

50 890 


14.0 
19.9 
23.0 













I 



Merica, Wallenberg,'] 
Scott i 



Heat Treatment of Duralumin 



277 



All of the alloys except those containing no copper (Nos. C2 
C4, and C9) show an increase of hardness of the hest-treated speci- 
mens over that of the annealed samples. The increase of hardness 
in those alloys containing copper, but no magnesium, is smaller 
than that in those containing both, but is quite definite. This is 
shown in the following table : 



Number of alloy 


Increase of ten- 
sile strength 

of heat-treated 

alloy (510° C) 

over annealed 

alloy 


Alloys containing no copper: 

C2 


Per cent 

+ 2 


C4 . . . 


+ 3 


C9 


— 4 


Alloys containing copper but no magnesium: 

C3 


-f-30 


C5 


+36 


Alloys containing both copper and magnesium: 

CI 


45 


Cll 


56 


C12 


110 







It is noticed that the best mechanical properties are produced 
by quenching from the higher temperatures (500 to 525° C). This 
is shown in Table 3, giving further data on two alloys, C8 and 
Cii, and will be shown more clearly below. 
121040°— 19 2 



278 Scientific Papers of the Bureau of Standards [Voi.ts 

TABLE 3. — Effect of Quenching Temperature on Tensile Properties 





AUoyC8 


Alloy Cll 


Quenching 


Aging 


Mechanical properties 


Aging Mectianical properties 


temperature 


20° C 


110° C 


Sclero- 
scope 
hard- 
ness 


Ultimate 
tensile 
strength 


Elonga- 
tion in 
2 inches 


20° C 


110°C 


Sclero- 
scope 
hard- 
ness 


Ultimate 
tensile 
strength 


Elonga- 
tion in 
2 inch en 


371° C 


Days 

13 

13 

7 

I 7 

f ^^ 
13 


Days 

6 
6 


1" 

12 
23 
23 

28.5 

1" 

22 

25.5 
25 

130-35 


Lbs./m.! 
27 260 
1 26 220 
J 29 130 
1 29 130 
J 39 540 
1 40 160 
J 41 200 
1 41 200 
( 46 400 
1 47 030 
J 48 900 
1 47 650 
f 47 230 
1 47 860 
1 44 130 
1 44 910 
J 49 680 
1 51 530 
45 160 
44 720 
44 720 
y 44 070 
J 46 520 
1 46 030 
53 440 
1 53 000 
f 34 960 
1 40 570 


Per ct. Days 
16.0 13 
18.0 13 


Days 
18 


Lbs./in.5 
J 35 900 
1 36 550 
J 35 020 
1 35 020 
f 43 790 
I 43 360 
( 44 010 
43 580 
( 50 450 
I 48 950 
f 51 740 
1 50 880 
f 52 590 
1 52 590 
f 51 520 
1 SO 870 
f 54 740 
1 55 590 


Peret 

19.0 
17.0 


422° C... 


18.5 
20.0 
12.0 
13.5 
17.0 
22.0 
19.5 

20.0 
22.0 
18.5 
20.5 
24.5 
23.0 
19.5 
17.0 
26.0 
25.0 


7 
7 

13 

13 

7 

7 

13 

13 

7 

7 

11 

11 

11 

11 

8 

8 


6 
6 

6 
6 

6 
6 

3 
3 


17 
25 

28 
31 

1 29.5 

1- 34 
1 ^ 


21.0 
21.0 

15.5 

18.5 


478° C 


7 6 
I 7 6 

13 

13 


23.5 
24.0 

22.0 


500° C 


7 
I 7 

J ^^ 

1 11 

11 

11 

8 

I 8 

( 11 


6 
6 

3 
3 

3 
3 


22.0 
22.0 

20.5 


510° C... 


21.0 
21.0 
24.0 




23.0 
20.0 


1 11 










520° C 


1, 








i 






21.0 
22.0 
23.0 
25.5 
21.0 
19.0 
5.0 
9.0 






i 11 












11 












11 












"5°C , 8 












1 8 












533° C 


1 ^^ 
I 13 


13 
13 




33 


J 48 370 
I 47 060 


9.5 
10.0 



Not only does the hardness increase after heat treatment, but 
so also does the ductility, as evidenced by the elongation in the 
tensile test. This is shown in Tables 2 and 3. 

1. EFFECT OF QUENCHING TEMPERATURE 

In Fig. I are shown the scleroscope hardness values of Cii 
quenched in water (20° C) from different temperatures and aged 
at room temperature for periods of time from a few hours to 30 
days. The form of these aging curves is similar to that shown by 
Cohn (see notes 3 and 5 on p. 271) ; that is, the hardness increases 
after quenching, at first rapidly and then more slowly. It is fur- 



W/IOKtSS^SMS- 



Merica, Wallenberg,! 
Scolt i 



Heat TreatTuent of Duralumin 



279 



«0 




^ "^ "^ ? 

P^' c^ t\i "V) 



28o 



Scientific Papers of tlw Bureau of Standards 



[Vol. IS 



ther ex-ident that the maximum hardness attained increases with 
the temperature up to approximatelv 520° C. 

The effect of quenching temperatiu^e is also sho'VNTi ven' nicely 
in an experiment of which the results are shown in Fig. 2. Two 
strips of 0.087-inch sheet of alloy X34 were used. The strip was 
placed in the furnace for heating in such a manner that a nearly 
Hnear temperature gradient existed between the two ends, as 







s 

"3 



i 



s 






o 



•^3uyu/a// SutAJjuSo^ - 9s^su/:?^o/y S(^0OSO^9pQ 



shown by thermocouples placed along the strip. Upon attaining 
the desired range of tempera tiures, the strip was quenched in 
boiling water and aged 20 hoiirs at 110° C. The scleroscope 
hardness was then determined along the axis of the strip, and is 
sho%vn in Fig. 2 as a fimction of the distance from one end of the 
sample. The distance may be regarded as a rough temperatiare 



^mm 



Merka, Wallenberg.-^ fj^^f Treatment oj Duralumm 281 

scale, the outside temperature limits having been determined and 
marked on the curve. One strip was quenched when the two 
ends were at 520 and 280° C, respectively; the other, when the 
ends were at 490 and 210° C, respectively. Beginning at about 
300° C, the effect of increased quenching temperature, other fac- 
tors remaining alike, is to increase the hardness after aging until 
a temperature of about 520° C is reached. Beyond that temper- 
ature the hardness again decreases ; the material becomes covered 
with a dark gray oxide coating and generally also with blisters, 
marking the temperature of eutectic melting. The effect of heat- 
ing to temperatures around 300° C is chiefly to anneal the specimen 
and to give lower values of the hardness (minimum on the curve) 
than is given by heating at lower temperatures. 

2. EFFECT OF AGING TEMPERATURE 

In Table 4 are given results of tests showing the effect of tem- 
perature of quenching bath and of aging carried out in the bath. 
The samples used were strips of A1-12 quenched from 520° C. 
The increase of strength with time of aging is evident. 



_^ 



282 



Scientific Papers of the Bureau of Standards 



[Vol. ts 



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Merica, Waltenberg, 
Scott 



Heat Treatment of Duralumin 



283 



A more complete picture of the phenomenon of hardening by 
aging at different temperatures is obtained from Figs. 3, 4, and 5, 
based upon data obtained on specimens of N34. The sclerocsope 
values of Fig. 3 were obtained upon samples quenched in boiling 
water from two temperatures, 515 and 525° C, and aged at differ- 
ent temperatures. The same figures are replotted in Fig. 4 in 
different form. 




T//*r^ o^ ^ytny /n J7oyj 



Fig. 3. — Effect of aging at different temperatures on the scleroscope hardness of samples 
quenched from 51 ^°C and 52 ^°C. {Alloy N-j^) 

It is noted (i) that the rate of hardening increases as the tem- 
perature of aging increases, (2) that the maximum hardness is 
obtained by aging at temperatures above 100° C, and (3) that at 
aging temperatures above 140° C the hardness eventually drops 
after reaching its maximum. 



284 



Scientific Papers of the Bureau of Standards 



[VoLis 




'^i>u/u^o/y Bu/X^/u^£?^ - ss9U/:?^e>^ in/oc>s^o^<^/y^ 



Merica, WaltenbergA 
Scott J 



Heat Treatment of Duralumin 



285 



Fig. 5 shows the results of an experiment similar to that of 
Fig. 2. The strips were quenched from 515° C in boiling water 
and aged for 20 hours thereafter in a furnace giving a temperature 
gradient from one end to the other of the sample. For a time of 
aging of 20 hours the hardness first increases with the temperature 
to a maximiun at about 180° C and then decreases. Above this 
temperature annealing sets in. 










3. EFFECT OF TEMPERATURE OF QUENCHING BATH 

Table 5 shows the effect of temperature of the quenching bath 
upon samples of A1-12 quenched from 520° C. 

It is noticed that the tensile strength of the alloys, as well as the 
elongation, increases with the time of aging. There is no marked 
121040°— 19 3 



286 Scientific Papers of the Bureau of Standards \Voi. is 

effect of the temperature of the quenching bath indicated in these 
results. Those samples quenched to 150° C gave practically the 
same results as those quenched to 230° C, although there is a 
slight improvement in the tensile properties of those quenched to 
150° C over those quenched to 100° C. 

In Table 6 are shown results of tests to determine the effect of 
aging at room temperature after aging at the temperatiore of the 
quenching bath. It will be noted that there is only a slight 
increase in the strength of the alloy produced by aging at 20° C 
after aging at the temperature of the quenching bath. 



Merica, Waltenberg,~\ 
Scott J 



Heat Treatment of Duralumin 



287 






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Mercia, Weltciiberg, ~] 
Scott J 



Heat Treatment of Duralumin 



289 



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Scientific Papers of the Bureau of Standards fvoi.15 







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M erica, WalienbergA 
Scott J 



Heat Treatment of Duralumin 



291 




»o 






I 



292 



Scieniific Papers of the Bureau of Standards 



[va. IS 




V) 
SO 



^^€C?fe£f to fe/Tiperature /na^/'catea/, <fuenchec/ 
/n jbo/'/m^ plater, a^ec^ y ofays art /JO^C 

O Heart ec/ /-oS/^^C^ coo/ffd to temperafur-e 

a^ed ^/ <days at /ZS°C 

5S0 500 ^50 ^00 J SO .300 Z 

(^uenchin^ Temperature "C 

Fig. %.— Comparison of scleroscope hardness of specimer^ of Alloy N-34, (i) heated to 
quenching temperature, quenched and aged 7 days at ijo° C, arui {2) heated to ^if'' C, 
cooled to quenching temperature, quenched and aged 21 days at I2§° C 



Sc^u"' ^'^^"'^'"'^'] Heat Treatment of Duralumin 293 

4. EFFECT OF PRIOR HEATING AT THE QUENCHING TEMPERATURE 

Fig. 6 shows the results of hardness measurements on samples 
held at the quenching temperature for varying periods of time 
quenched and aged at 170° C. The samples held from 5 to 180 
minutes at the quenching temperature give values of the hardness 
differing by less than the probable error of measurement. The 
low values found on the sample held 21 hours are due probably 
to the blistering which was noticed in the sample. 

5. EFFECT OF PREHEATING TO SlS° C BEFORE QUENCHING FROM 
LOWER TEMPERATURES 

In Figs. 7 and 8 are shown the results of experiments to deter- 
mine whether preheating to a temperature higher than the quench- 
ing one before quenching gave a different hardness than by heating 
merely to the quenching temperature. 

Although owing to a slight difference in the aging conditions the 
comparison is not quite definite, it is obvious (i) that the hardness 
obtained by heating to 515° C, cooling to a temperature, i(when 
i<5i5° C), and quenching is always greater than that obtained 
by quenching from t°, and (2) that whether the specimen is pre- 
heated or not to a higher tem.perature before quenching frora some 
lower temperature the hardness obtained increases with higher 
quenching temperatures. 

IV. MISCELLANEOUS TESTS 

In Table 7 are shown the results of a number of tests of alloy 
N34, including determmations of the proportional limit of several 
heat-treated samples. 



294 



Scientific Papers of the Bureau of Standards 



\Vol. IS 



TABLE 7.— Effect of Varying Aging Temperature and Time of Aging on the Tensile 
Properties of Aluminum Alloy Sheet « (Alloy N34) 













Sclero- 










No. 


Thick- 
ness 

of 
sheet 


Heat 

treatmeni 

quenched 

from — 


Aged 
at— 


Aged 


scope 
hard- 
ness 
magni- 
fying 
ham- 
mer 


Ultimate 
strength 


Propor- 
tional 
limit 


Elon- 
gation 

in2 
inches 


Remarks 




Inch 


' C 


°C 


Days 




Lbs./ln.2 


Lbs./in.2 


Per ct. 




14 


0.034 


515 







26 




17 000 


5 


Broke at extensometer contact* 


15 


.034 


515 


...... 





26 


38 200 


17 000 


6 


Do. 


24 


.034 


515 


105 


2 


39 


52 900 


27 000 


7 


Do. 


25 


.034 


515 


105 


2 


39 


54 900 


29 000 


7 


Do. 


26 


.034 


515 


105 


4 


39 


55 500 


27 000 


9 


Do. 


27 


.034 


515 


105 


4 


39 


55 200 


26 000 


8.5 


Do. 


28 


.034 
.034 


515 
515 


105 
105 


7 

7 


38 
38 


63 200 
56 200 




23 
9 


No extension measurements. 


29 


29 000 


Broke at extensometer contact. 


30 


.034 


515 


105 


9 


42 


61 200 


30 000 


16 


Extensometer attached to flat sur- 
faces. Broke at gage point. 


31 


.034 


515 


105 


9 


41 


60 600 


29 000 


14.5 


Do. 


32 


.034 


515 


105 


14 


40 


62 200 


31 000 


21 


Extensometer attached to flat sur- 
faces. 


33 


.034 


515 


105 


14 


41 


61 600 


33 000 


23 


Do. 


16 


.034 


515 


125 


2 


41^6 


49 500 


27 000 


2.5 


Broke at extensometer contact. 


17 


.034 


515 


125 


2 


42-50 


54 500 


32 000 


3 


Do. 


18 


.034 


515 


125 


4 


45-50 


58 200 


30 000 


3 


Do. 


19 


.034 


515 


125 


4 


45-50 


58 500 


33 000 


4 


Do. 


20 


.034 


515 


125 


7 


44-47 


60 000 


39 000 


4.5 


. Do. 


21 


.034 


515 


125 


7 


42^8 


58 000 


37 000 


4 


Do. 


34 


.087 


515 


125 


7 


46-50 


61 200 


32 000 


18 


Extensometer attached to flat sur- 
face. 


35 


.087 


515 


125 


7 


47-52 


62 000 


31 000 


18 


Do. 


22 


.034 


515 


125 


14 


47-52 


64 900 


37 000 


11 


Do. 


23 


.034 


515 


125 


14 


47-54 


61 500 


35 000 


3 


Extensometer attached to flat sur- 
ace. Fractured at blister. 


2 


.034 
.034 
.034 
.034 


515 
515 
515 
515 


150 
150 
150 
150 


2 

2 
4 
4 


50 
49 
50 
50 


51 110 
50 940 
59 800 
61 500 




16 
16.5 

11.5 
6 


No extension measurements. 


3 




Do. 


4 




Do. 


5 


41 000 


Broke at extensometer contact. 


6 


.034 


515 


150 


6 


50 


63 800 




6 


No extensometer measurements. 


7 


.034 


515 


150 


6 


50 


62 200 


43 000 


6.5 


Broke at extensometer contact. 


1 


.087 
.034 
.034 
.034 
.034 
.034 
.034 


515 

515 
515 
515 
515 
515 
515 


170 
170 
170 
170 
170 
170 
170 


2 
2 
2 
2 

4 
4 
4 


50 

51.5 

51.5 

51.5 

44 

44 


63 900 
51 420 
51 850 
50 760 
58 200 
55 500 
57 300 




10 

10.5 

10 

9.5 

9.5 

5 

6 


No extension measurements. 


8 




Do. 


9 




Do. 


10 




Do. 


11 




Do. 


12 




Do. 


13 


34 000 


Broke at extensometer contact. 



"■ Where two values of the hardness are given the lower one shows the hardness of the end near the door 
of the furnace in which the sample was heated for quenching, and the other value is the hardness of the 
opposite end, the difltercnce in hardness being the result of a temperature gradient in the furnace. The 
specimens in this condition all broke at the soft end and hence their tensile properties are hardly as high 
as can be expected of the material. 



Merica, Wallenberg,' 
Scolt 



] Heat Treatment of Duralumin 



295 



Inasmuch as so many of the tests made dm-ing this investigation 
were measurements of scleroscope hardness, a comparison was 
made between the hardness and the tensile proportional limit of 
some specimens of N34 during aging at two temperatures. The 
results are shown in Fig. 9, and it is noted that the curves in each 
show quite close parallelism. 



^,000 




/a a 00 



zo 



* n (Juenc/7,^e/ from S/S°C /n jbo/'/tn^ kratei', a^ec/ af /2S°C 

^ 'l' ,. .. „ ., /as ° 



Q /y^r'e/ne.ss 

• Pi'ofiofi'-/o/7a/ t I'm//- 



iZ 



/•4 



Fig. 9.- 



Z 4- c & /o 

7~/me of /J^/n^ /n I^ay^S. 
Comparison during aging of scleroscope hardness and tensile proportional limit 
{Alloy N-34) 



Experiments were conducted on specimens of A 1-12 to ascer- 
tain whether the hardening during aging could be hastened by 
vibration. The results of several such tests in which the vibration 
was produced by a bell clapper indicated that there was no differ- 
ence in the rate of hardening between vibrated and quiet specimens. 



296 



Scientific Papers of the Bureau of Standards 



[Vol. IS 



1. DENSITY AND DILATATION 

The density was determined of samples of N34 in different con- 
ditions, and Table 8 gives the results of these tests. In some cases 
one dimension of the specimen was determined also, and its changes 
recorded in column 4 of the same table. The changes in density 
are quite small as the material undergoes heat treatment or anneal- 
ing, except when the temperature exceeds from 520 to 530° C, the 
temperature of eutectic melting, when a marked increase in length 
is noted. 



TABLE 8.— The Density and Length Changes in Duralumin (N34) 



Sample 



W34D1.. 
N34D1.. 
N34D2.. 
N34r)3.., 
N34 D3-a, 
N34 D3-a 
N34D4.. 
N34 D4-a 
N34 D4-a 
N34D5.. 
N34 r>5-a 
N34 D5-a 



Treatmeat 



Quenched; not aged 

Same as above, after aging at 150° C . 

Annealed, after roiling, at 515° C 

As rolled, 0.033 inch thick 

Same after annealing at 500° C 

Annealed at 530° C 

As rolled, 0.088 inch thick 

Same after annealing at 500° C 

Annealed at 530° C 

As rolled, 0.25 inch thick 

Same after annealing at 500° C 

Annealed at 530° C 



Density Length 



2.762 
2.762 
2.759 
2. 754 
2.742 



2.750 
2.747 



2.764 
2.762 



Inches 



12.014 

12. 024 

12.0477 

11.982 

11.982 

11.9973 

11.9954 

11.9963 

12. 0019 



Merica, Waltenberg, 
Scott 



Heat Treatment of Duralumin 



297 



The linear expansion up to 520° C was determined on two bars 
of N34, one as rolled, the other after heat treatment, consisting of 
quenching from 520° C and aging two days at 1 20° C. The expan- 
sion curves are given in Fig. 10 and show irregularities in the 
neighborhood of 300° C. 




O 100 200 JOO ¥00 SOQ iiOO'C 

Fig. 10. — Linear expansion of N-J4, o to 500° C 

2. THE ELECTRICAL RESISTIVITY 

Electrical resistivity measurements were made in vacuo over the 
temperature range o to 530° C by the method described by 
Burgess and Kellberg ^ on 0.25 mm wire drawn from a cylinder 
cut from 34^-inch sheet of Cii. It was necessary, however, to 
bring both of the aluminum-alloy leads out of the thermometer, 
as it was impossible to weld them to platiimm. The data obtained 



' Scientific Paper of the Bureau of Standards, No. 336; 1914. 



298 



Scientific Papers of the Bureau of Standards 



[Vol. IS 



from the first run are plotted as resistance of aluminum alloy 
against temperature in Fig. 1 1 . 

The change in direction of the resistivity cur\^e at about 300° C 
is quite evident and indicates a change in the constitution of the 
alloy. It is evident both on heating and cooling, although a 



^.a 



/.s 






E^/ectr/cal 



C// 

T^es/sfance o. 



■ JDara/u/n/n 







/.z 






0.8 



COO//r7<3 "~* 

from ^'^O'C 






as^ 



/oo 



'^OO 



200 JOO 

Fig. II. — Electrical resistivity of C-ii, o to 520° C 



500 'C 



change in resistivity at room temperature has taken place, resulting 
from the annealing produced during the series of measurements. 

Following this run the material was heated to 440° C in its tube 
and cooled in air. The cooling was fairly rapid, as the outside 
diameter of the quartz tube was only 8 mm. The tube was then 



>S 



Merica, WaUenberg,"] 
Scott J 



Heat Treatment of Duralumin 



299 



put in a steam bath and resistance measurements taken, as shown 
in the table below : 



Time la steam balh 


Pt re- 
sistance 


Al re- 
sistance 


Time in steam bath 


Pt re- 
sistance 


Al re- 
sistance 




Ohms. 

1. 7302 
1. 7302 
1. 7302 
1. 7297 
1. 7301 


Ohms. 
0. 9035 
.9047 
.9051 
.9054 
.9060 


4J^ hours 


Ohms. 

1. 7300 

1. 7301 
1. 7301 
1.7298 


Ohms. 

0. 9063 


i/^ hour .' . - 


6 hours 


.9069 


1 hour 


7 hours 


.9068 


2 hours 


11 hour 


.9069 













The specific resistance of this alloy was determined on a wire 
drawn to 2.54 mm diameter and annealed at 400° C. It was 
foimd to be 3.35 microhms per centimeter cube. 

V. MECHANISM OF HARDENING DURING AGING AFTER 

RAPID COOLING 

Apparently no attempt has ever been made to develop an expla- 
nation for the changes in the physical, particularly mechanical, 
properties of this alloy during aging after rapid cooling. The 
changes which take place are quite marked and definite and must 
correspond to some quite as definite changes in the structure and 
constitution of the alloy, or at least to profound molecular changes. 
If we are not able to show that actual phase changes take place 
during aging, we must then ascribe these changes in physical 
properties to alterations in the atomic or molecular structure. 

All of the evidence which the authors have been able to find or 
to accumulate seems to indicate that the hardening during aging 
is actually accompanied by a phase change within the alloy. 
In so far as it can be said then that this phase change causes the 
hardening, for the reason that it accompanies it, this phase change 
may be regarded as its active cause. 

Elsewhere ^ the authors have determined the solubility at differ- 
ent temperatures in aluminum of CuAla and of Mg^Alg, the alumi- 
num-rich compounds of the copper-aluminum and magnesium- 
aluminum binary alloy series, respectively. The solubility-tem- 
perature curves of these compounds are reproduced in Figs. 12 
and 13; the solubility of both compounds diminishes rapidly with 
lowered temperature. 

• p. D. Merica, R. G. Waltenberu, and J. R. Freeman, jr.. The Constitution and MetalloKrapUy of Alu- 
minum and its I.isht Alloys with Copper and with Magnesium, Scientific Paper of the Burcuu of ,Staud- 
ards, No. 337, 1919; also Bull. A. I. M. B. No. 151, p. 1031, 1919. 



300 



Scientific Papers of the Bureau of Standards 



[Vol. IS 



600° 



5oo° 



o i^oo» 



JOO" 



3 

\ eg 

\ \v liquid o / 


& 


liquid 


\v^^ solid solution*ljquid — | 


\ Bolld ^s. '\j ~ 
\ solution \_^ -^''^^ 

^\ liquid \^^,^< Cu^lp /, 


solid solution^ 

of CuAl2 
in aluminuu 

o 


-->. 


solid eolation + CuAlj ' j 






/ 


EOlid solution 

+ 




/ 


CuAl2 




C A X 
O /x X 


X 


o ...Ko CuAlj observed 
after annealing 

X . . .CuAlj observed 

after anneal ing 




1 1 
1 1 


/ e 

1 


1 1 



40 2.. 

Percentage of copper by weight 



Fig. 12. — Portion of equilibrium diagram of copper-aluminum alloy series shouuing solu- 
bility curve be of CiiAi^ in aluminum 



600 



5oo 



too 



joo 



a 










^^^"""--^ 






\ ^""~^-v^^^ liquid 


*^ \ liquid 




^^--.^ 


\ \ 

\ solid \ 
\ solution \ 

\ * \ 

V liquid \c 

b ] ^^-^ 

solid BolutlorJ 


< 

i- 

---"d 


\ soll< 
\ 

\ 

\ 

N 
\ 

solid 
solution 
of L'e^Al, 
in aluminum 


1 solution ^^^^^ 
+ liquid ^\^^ 


OC X x« 


/ «8l*"j 




y^ solid solution 


/ 


- 


/* X t n 


e 
1 I 




/ 




1 1 


o ...No %i,Al observed 

after annealing 
X ...1>E!4-*13 observed 

1 after annealing 



1^0 5 10 i5. 

Pere.entage of magnesium by weight 



20 



25 



Fig. 13. — Portion of equilibrium diagram of magnesium^aluminum alloy series shoiLiiig 
solubility curve be of Mg^Ai^ in aluminum 



Merica, WaHenberf,! 
Scolt J 



Heat Treatment of Duralumin 



301 



Upon slowly cooling, an alloy containing 3 per cent of copper 
from 500° C, the CuAlj precipitates from solid solution to main- 
tain equilibrium along the line he. The alloy so obtained is soft 
and does not harden upon aging. Rapid cooling of the same alloy 
from 500° C by quenching partially or wholly suppresses this pre- 
cipitation of CuAlj. If the alloy is held at a low temperature, such 
as at that of liquid air ( — 180° C) , no further hardening takes place 
upon aging. The alloy is not in equilibrium, but the rate of 
nuclear formation of CuAlj is so small that no CuAlj precipitates 
to bring about equilibrium. If, however, the temperature of the 
alloy is raised to 100° C, or even to ordinary room temperature, 
according to the theory which the authors propose, the mobility 
of the molecules becomes sufficiently great that precipitation of 
the CuAlj takes place in the form of very fine particles of colloidal 
dispersion. To this precipitation is due the hardening during 
aging of duralumin. 

The evidence in favor of this theory is largely of an indirect 
nature, the only direct confirmation of its truth being furnished 
by the results of thermal analysis. 

Upon heating a specimen of duralumin which has been quenched 
from 500° C but not aged, an evolution of heat occurs at from 250 
to 275° C. This is shown in inverse rate-reading curves of three 
compositions, C8, Cii, and N34, in Figs. 14, 15, and 16. No 
thermal change takes place upon cooling the same specimen, 
provided it has not been heated beyond 520° C. Upon reheating 
the same slowly cooled specimen no evolution of heat is found cor- 
responding to that upon the first heating. Without doubt, there- 
fore, a chemical reaction takes place at 250 to 275° C upon heating 
the quenched sample with evolution of heat; that is, indicating 
the formation of stable from unstable phases, not a transformation 
of stable to other stable phases, the two systems being in equilib- 
rium during the transformation. Such a transformation must 
take place with heat absorption upon raising the temperature. 

A specimen which has been quenched and aged at from 100 to 
150° C to secure maximum hardness shows little or no evolution 
of heat upon heating. (See curves in Figs. 14, 15, and 16.) What- 
ever the chemical reaction be that is indicated on the first heating 
curve of a quenched specimen, it has taken place during the aging 
of the specimen, during which the hardening also occturs; stable 
phases have formed, and the subsequent heating curve shows no 
arrest corresponding to that of the quenched specimen. 



-rrr 



3o: 



Scientific Papers of the Bureau of Standards 



IVol. IS 



Q 






f^ 



soo' 



^00' 



300' 



ZOO" 



C 8 



C e D 



/st^Up 



/st JDot^n 



Znd Up 



10 

I 



Time irt seconds 

ZO JO I 



J_ 



1st Up 



is eo 



Xn yerse Ba te Cu n^£5 

Fig. 14. — Heating and cooling curves of C-8. First run up showing arrest at 300° C was 
taken three hours after quetiching. C-8-D is a curve obtained on a quenched sample after 
aging 18 months at 20°C 



Merica, Wallenberg,! 
Scott J 



Heat Treatment of Duralumin 



303 









c u 








. 


A 






r 
; 




•> 


: 




1 


i 

• 




_i 


i 




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• 


:• 


^ 


ZOO" 




/ 




•'.. 




up 


^ TDoyvn 


Up 


Down 








Time in seconc/s 






JO 

1 


IS ts 

1 1 


zo 

1 


10 IS 

1 1 


lO IS 



XfiYcrse IHote Curyes 

Fig. 75. — Heating and cooling curves of C-ii. First run uf> on C-ii-A shows an arrest 
at 26^° C in a sample which had been quenched and then aged 12 days at 20° C. Second 
■• run up, marked C-ii-A , shoivs no arrest in a sample which had been quenched into boiling 
water and aged 10 days at 120° C 



304 



Scientific Papers of the Bureau of Standards 



[Vol. IS 



pi 



^00° 



300° } 



ZOO 



1st Up 

Quenched 
net aijed 



1st Up 
Ouenchea ae^ed 



N3H 



soo 



war 



300 



zoo 



Up Tlorrn 

Quenched 
not a^ed 



time m seconds 
zo is 



Up 

Blanhr 



J L 



Inverse 'Rate Cury^es 
Fig. i6. — Heating and cooling curves of N-J4 showing inverse arrest in quenched samples 
■which had not been aged but no arrest in samples which had been aged after quenching 



mnca, waiunberg,-^ ^icat Treatment of Duralumin 



305 



SCO 






Q) 



H!? 



^fOO 



30(f-: 



N ZQ' 



•-sia 



10 IS 



■ - S02' 






-383* 



-SOZ' 



ma' 



\- ts<,* 



.• -3iS' 



/St Up 1st Uon-n 2nd Up 2nd Uoyrn 



Time iri seconds 

20 SS lO /S 



IS" 20 



Fig. 17. — Heating and cooling curve of N-28. {Cu 4.Q8 per cent, Mg. 2.41 per cent) 



3o6 Scientific Papers of the Bureau of Standards [Voi. is 

This chemical reaction can hardly be other than the precipita- 
tion of CuAU from its supersaturated solution in aluminum, 
although direct visual evidence bearing on this question is also 
lacking. In describing the attempt which was made to recognize 
microscopically the phase change during aging just predicated a 
digression must be made in order to discuss the general features 
of the microstructure of duralumin, which has apparently not 
been done before. 

1. STRUCTURE OF DURALUMIN 

This microstructure may be developed either by etching in a 
relatively concentrated solution of sodiimi hydroxide, NaOH, a 
dilute solution of hydrofluoric acid, HF, or in a dilute solution of 
NaOH. The grain structure of the alloy is best developed by the 
two former solutions; lo per cent NaOH and 5 per cent HF are 
generally used for this purpose. For the identification and study 
of the different microscopic constituents of the alloy a o.i per cent 
solution of NaOH has, however, shown itself much superior to the 
former ones, and this solution has been used in most of the authors' 
investigations. 

Duralumin after rolling shows a structure similar to that in Fig. 
18, which is quite typical. Fig. 19 shows the same alloy at a 
higher magnification. Grains of aluminimi (in which are dissolved 
Si, CuAlj, and Mg^Alg) are surrounded by strings of islands of 
eutectic (CuAlj aluminum, FeAlg aluminum, and possibly others), 
which are white in Fig. 18. Upon examination under a higher 
power the eutectic is seen to consist of two constituents, one of a 
brownish color, the other white. These two constituents are evi- 
dent in Fig. 19. In another article by the authors (see note 9, on 
p. — ) these two constituents have been identified as FeAlg (brown) 
and CuAlj (white) , respectively. Quite often, but not always, the 
FeAlg surrounds the CuAlj, as is shown in the figure. 

Besides these two constituents a third, of pronounced bluish 
color, is visible. This is readily distinguished im.der the micro- 
scope; not always so readily in a photograph. It is seen within 
an island of CuAlz in Fig. 20. In the same article the authors 
have expressed the opinion that this is MgjSi; it only occurs in 
alloys containing magnesium. 

Upon still closer observation the grains of altuninimi solid solu- 
tion are seen to contain minute particles of a constituent. These 
are shown in Figs. 21 to 25. These particles are so small that it 
is impossible to identify them with certainty. Inasmuch as they 



Scientific Papers of the Bureau of Standards, Vol. 15 



'm 



Fig. 



i8. — Rolled duralumin, N 34. Etched with o.i per cent 
NaOH. Xioo 




Fig. 19. — Rolled duralumin , N J4 (E4). Etched with o.i per cent 
NaOH. Xiooo 



Scientific Papers of the Bureau of Standards, Vol. 1 5 




Fig. 20. — Specimen of N 28 containing Cu and Mg, showing island 
of MgjSi {dark) within one of CuAh (white). Xiooo 




Fig. 21. — Sample of duralumin, E j, showing FeAl^ and CuAU 
eutectic, arid fine particles throughout ground mass, y.1000 



Scientific Papers of the Bureau of Standards, Vol. 15 




Fig. 



-Sample of E J, same as Fig. 21. X2000 



.U 



^■' r-,r- 












i ilf. _-j. — Sample of duralutnin E J-/' , (i//t/ ainiealiiig JO hoiiis at 
$00" C, quenching and aging at room temperature. Y^IOOO 



Scientific Papers of the Bureau of Standards, Vol. 15 







I. 




» k -••,'«'. ';..• "Vl'iV.'Ba-<a"<K;ji.>i3S^«\f4'.-*!Vv;l'lC 



,. v.-t-cj af 



Fig. 24. — Same material as in I iy. Jj. Y-^iooo 




M erica, Wallenberg.-^ Heat Treatment of Duralumin 307 

occur also in altiminum itself, they must consist in part, at least, 
of the compound X (of iron, silicon, and probably also aluminum; 
see note 9, on p. 299) and possibly FeAlg; probably CuAlz is also 
present in this form. All of this generation of particles have 
undoubtedly separated during cooling from a solid solution in 
aluminum at higher temperatures. 

The visible structure of duralumin changes but slightly upon 
heat treatment. Rolled duralumin consists of elongated grains. 
Upon heating such material to 500° C recrystallization of the 
aluminum solid solution grains first occurs, and the fine grains so 
formed increase in size. This growth is nattually interrupted by 
quenching. Immediately after quenching, therefore, the grains 
may be either larger or smaller than the original ones, depending 
upon the period of heating at 500° C and the rate of heating to 
that temperature. During subsequent aging the grains do not 
change in size. Heating to 500° C also results in the solution of 
some or all of the CuAlj eutectic grains seen in the rolled material 
to correspond to equilibrium. The FeAlj does not dissolve. 

If there occurs druing the aging of duralumin after quenching 
a gradual precipitation of CuAlg particles to correspond to its 
diminished solubility at the lower temperatures, one would expect 
to be able to observe some difference between the microstructure 
of the quenched unaged specimen and that of the quenched one 
after thorough aging. The particles of CuAlj may quite well be 
too small to be resolvable microscopically, but the presence of a 
large number of such colloidal particles might be expected to 
accelerate the etching of the specimen; at least troostite etches 
much more readily than martensite or sorbite, and it is considered 
quite generally to consist of a colloidal solution of FcgC in alpha 
iron. Samples of N34, some of which had been heated at 500° C, 
quenched in water and immediately etched, and some of which 
had been subsequently aged at 130° C after identical treatment to 
develop maximum hardness, were carefully compared in their 
appearance after etching in the same solution (o.i per cent NaOH) 
and for the same periods of time. No difference was observed in 
the- structure nor in the general shades of the etched surfaces of 
these two groups of specimens. 

The authors have to date, therefore, no direct structural evi- 
dence of the precipitation of CuAl^ during aging of duralumin. 

A difference in the rate of etching of quenched unaged and of 
quenched and aged may quite possibly be obscured by the pres- 



3o8 Scientific Papers of tJie Bureau of Standards [Voi. is 

ence of other constituents in fine dispersion, present in both cases. 
It was noted above that there are always present a number of 
fine particles of the X constituents. A structural study of duralu- 
min made with pure aluminum free from iron and siUcon might 
3'ield more positive results. 

It is interesting to note that although the velocity of nuclear 
formation of CuAU at temperatures from 20 to 400° C seems to 
be quite normal, judging by thermal analysis, the velocity of 
crystalUzation or of coalescence of the nuclei is e\-idently quite 
remarkably small. Thus it was fotmd (see note 9 on p. 299) that 
there was no visible precipitation of CuAl, in an alloy containing 
3 per cent of copper upon annealing at 300° C for 20 hours after 
obtaining all of the CuAl, in solution by annealing at 500° C. 
Only by ver}' slow cooling from 500 to 20° C could a visible pre- 
cipitate of CuAlj be produced. Slow velocities of cr}-stallization 
seem to be characteristic both of CuAl^ and of aluminum. 

Although it can not be directly proved that the thermal arrest 
at about 250° C, noticed upon heating a quenched imaged speci- 
men of duralumin, is due to the precipitation of CuAl,, no evidence 
directly contradicts this assvunption, which is in entire accord 
with our knowledge of the equilibrium within the alloy, and this 
arrest can not be assigned to any other phase change. 

It has been shown by many pre\-ious investigations and con- 
firmed by the authors that aluminum tmdergoes no transformation 
in the soUd state between ordinan- temperatures and its melting 
point. Xo other phase changes could occiu" in the main mass of 
duralumin, the grains of solid solution, therefore, except those of 
solution or precipitation of FeAlj, of the X compoimd, of CuAl,, 
of ^^Ig^Alj, or of Mg,Si within the grains. Aluminum, which con- 
tains the same amoimts of FeAlj and of the X compound as does 
duralumin, is not altered by heat treatment as is duralumin, nor 
does it show a reverse heat effect upon heating as does the latter. 
This heat effect must therefore be due to the precipitation either 
of CuAlj, Mg^Alg, or Mg,Si. But the alloys containing only mag- 
nesium in amounts up to 3 per cent also do not harden upon 
aging. There remains only the precipitation of CuAl, with which 
to explain this heat effect. 

The theor>^ outlined above of the mechanism of the hardening 
of duralumin during aging most readil}' explains the interesting 
fact discovered by ^Ir. Blough, and confirmed by the authors, 
that the amount of hardening during aging increases as the tem- 
perature of quenching increases. At higher quenching tempera- 



Merka. Wallenberg.! j^^^^ Treatment of Duralumiu ' 309 



tures more and more CuAlz is dissolved in solid solution. After 
quenching the CuAlj is in excess of its solubility; the higher the 
quenching temperatiue the greater is the excess, and this is pre- 
cipitated during aging. The hardening is in proportion to the 
amotmt of the highly dispersed CUAI2 formed. 

If this theory is accepted for the moment, it is interesting to 
consider the effect of degree of dispersion upon hardness in the 
case of a solid solution, in this case of CuAlj in aluminum. Dura- 
lumin immediately after quenching is generally softer than it is 
in the annealed condition. Thus, alloy Cii in the form of sheet 
gave the following values of hardness : Scleroscope hardness (mag- 
nifying hammer) : Annealed at 300°, 17; quenched, but not aged, 
16; quenched and aged 8 days, 35. This is probably due to the 
fact that a specimen as ordinarily cooled after annealing still con- 
tains some dissolved CuAlz in excess of its solubility ; the material 
hardens slightly during cooling. Specimens cooled extremely 
slowly give a scleroscope hardness of from 7 to 10, much lower than 
that of the quenched unaged ones. 

Upon aging a quenched specimen at 200° C, for example, the 
hardness first increases to a maximum and afterward decreases. 
During that aging there has been first a formation of fine nuclei 
of CuAlj followed by coalescence of these particles into ones of 
larger size. There is, therefore, a certain average size of particle 
of CUAI2, for which the hardness of the material is a maximum; 
atomic dispersion of the solute, CuAla, is not the dispersion that 
produces the maximum hardness, but some intermediate one 
between it and that at which the particles become visible by ordi- 
nary means. 

It is interesting to observe that the properties of other light 
alloys of aluminum are influenced by heat treatment and aging. 
Thus Rosenhain and Archbutt ^^ have found that the tensile 
strength of sand-cast aluminum-zinc alloys increases upon aging. 
In another article (see note 7, on p. 272) by two of the authors it 
has been shown that whereas alloys of aluminum-magnesium, 
aluminum-manganese, aluminum-manganese-magnesiuni, and 
aluminum -nickel do not harden upon quenching and aging; those 
of aluminum-magnesium-nickel do. The solubility of zinc in 
aluminum decreases from 40 per cent at the eutectic temperature 
to about 25 per cent at 256° C and is probably much less at still 
lower temperatures. As in the case of the copper-aluminum alloys 

" Report to the Alloys Research Committee, Proc. Inst. Mech. Ijng., p. 319; 1912. 



3IO Scientific Papers of the Bureau of Standards [Voi. is 

decreasing solubility at lower temperatures of the constituent, 
CuAlj or zinc, is accompanied by the possibility of hardening by 
quenching and aging. 

Inasmuch as the aluminum-nickel-magnesium alloys also harden 
by aging, we may expect an appreciable solubility of NiAlg in solid 
aluminum at higher temperatmres. The solubility of MnAlj is 
undoubtedly qtiite low. 

2. ANALOGY BETWEEN THE HARDENING OF DURALUMIN AND THAT OF 

STEEL 

The hardening of duralumin upon the basis of this hypothesis 
presents an interesting analogy with that of steel. The hardening 
of steel is due to the partial or entire suppression of the eutectoid 
transformation. Most recent thought regards it as due more 
directly to the suppression of the cementite precipitation (as pearl- 
ite) , the transformation of 7 into a iron having taken place at least 
in part. The partial suppression, therefore, of the precipitation 
of a compound from a solid solution is common both to rapidly 
cooled steel and to duralumin. 

A sample of steel which has been hardened, but not tempered, 
shows an evolution of heat upon heating " through its tempering 
range exactly as does dtiralumin. This is due to the precipitation 
of FcgC in finely divided form in the case of steel exactly as it seems 
to be due to that of CuAlj in duralumin. 

During the tempering or aging of steel at from 100 to 300° C 
the hardness usually decreases immediately; that is, the maximum 
hardness of steel is obtained by quenching alone, whereas that of 
duralumin is produced after aging. In the case of some high- 
carbon steels (from 0.9 to 1.7 per cent C), however, the hardness 
increases during tempering after quenching exactly as in the case 
of duralumin.^^ The maximum hardness in hardened steel in 
creases with the carbon content, as it does in duralumin with 
the copper content. 

It has been found that tool steel containing tungsten undergoes 
an increase of hardness dm-ing tempering at from 400 to 650° C 
after quenching from 1350° C. " 

^' H.Scott, ESect of Rate of Temperature Change on Transformations in Alloy Steel, Scientific Paper 
No. 335, of the Bureau of Standards, 1919; also Bull. A. I. M. E. No. 146, p. 157; 1919. 
1' E. Maurer, Harten und Anlassen von Eisen und Stahl, Metallurgie, 6, p. 33; 1909. 
'3 Edwards and Kikkawa, Joum. Iron and Steel Institute, 92, p. 6: IQ15. 



Merica,waitenberg,-j j/^^^ Treatment of Duralumin 311 

3. EUTECTIC STRUCTURE AND INFLUENCE OF MAGNESroM 

There is one fact which is not readily explained by the author's 
hypothesis. Although alloys containing only magnesium and no 
copper do not harden and alloys containing only copper with no 
magnesium do harden, those containing both copper and magne- 
sium undergo a mtich greater hardening than do those with copper 
alone. Magnesium, therefore, exerts no effect by itself in this 
direction, and is not essential to the hardening power, but it mate- 
rially increases the effect of the copper. The hypothesis developed 
above does not indicate any reason for this effect. 

The authors are of the opinion that the influence of the mag- 
nesium is of a secondary nature. Thus it seems probable that some 
magnesium unites with the silicon present to form MgjSi, the blue 
constituent always foimd in alloys containing magnesium. The 
removal of the silicon in this manner may be the direct cause of 
the resultant increase of hardening effect. This would agree with 
the observed fact that with usual silicon content 0.5 per cent rnag- 
nesium is enough to fully develop the partially latent hardening 
power of the copper-aluminum alloys. The addition of more mag- 
nesium produces a somewhat harder alloy in all conditions, but 
does not materially increase the hardening effect. This is shown 
by the following comparison : 





Alloy 


Copper 


Magnesium 


Tensile strength 


Increase in 
tensile 




Annealed 


Hardened 


strength upon 
hardening 


cu 


Per cent 
2.6 
3.2 


Per cent 
1.3 

0.5 


Lbs./in.2 
35 000 
23 000 


Lbs./in.2 
56 000 
49 000 


Per cent 
60 


C12 


110 







Consideration of the test results of Table 2 shows that magnesium 
hardens the aluminum matrix considerably even in the annealed 
condition. It is probable that the alteration of this matrix affects 
markedly the dispersion of the precipitation of CuAla during aging 
and consequently the mechanical properties obtained. 

There is another feature of the structure of duralumin which is 
of great importance and in which may be found some part of the 
explanation for the effect of magnesium. This is the manner in 
which the FeAlj and the CuAlj eutectics crystallize. 



312 Scientific Papers of the Bureau of Standards \V0i.15 

There are several possible binary eutectics in duralumin, namely, 
the following : 



Eutectic 



Eutectic tem- 
perature 



FeAl3+ aluminum solid solution 

Si (cryst)+a]uminum solid solution 

X compound -r aluminum solid solution. 

CuAlc+aluminum solid solution 

MgiAb-f aluminum solid solution 

Mg~Sl+ aluminum solid solution 



°C 

640-650 
570-580 

610 
520-540 

450 

440 (?) 



The amounts, by volume, of the eutectics with FeAlj and with 
CuAlj in ordinary duralumin are fairly large and about equal, 
that with MgjSi somewhat less, that with X and with IMg^Alg 
usually almost nil. The approximate temperatures of eutectic 
solidification are given above ; they represent in all cases the tem- 
peratures observed in the presence of both the FeAlj and the X 
eutectic. The presence of CuAlj or ^Ig^Si lowers the eutectic 
temperatures of the other binary eutectics. Thus, in the presence 
of MgjSi, the eutectic temperatm-e of CuAlz-aluminum is reduced 
from 540 to 520-530° C, and this is always obtained as a thermal 
arrest in heating or cooling duralumin. 

The order of solidification of these binary eutectics in aluminum- 
rich alloys is a matter of the greatest importance. Fig. 26 shows 
the probable form of the equilibrium at the aluminum end of the 
ternary system, Al-Cu-Fe. An alloy containing about 0.5 per 
cent Fe and 3 per cent Cu (at g in the figure) would follow the line 
gf-jc upon solidification. A solid solution of aluminum with 
CuAlj (FeAlg is almost insoluble in aluminum) first cr\^stallizes, 
and the composition of the liquid changes along the curve gf with 
lowering of temperature. At / the binary eutectic FeAlg alumi- 
num solid solution crystallizes, and also along fc. The liquid 
remaining at / is contained in the interstices between the solid 
grains of aluminum solid solution, and the FeAlg crystallizes upon 
these grains at the boundary between solid and liquid. At c the 
binary eutectic CuAla aluminum solid solution also crystallizes 
with the remainder of the first eutectic. The resultant structure 
is shown in Figs. 19, 20, 21, 22, 23, and 24. The FeAlj often 
entirely surrounds and isolates the CuAlz crystals. 

When a specimen having such a structure is heated to 500° C 
for quenching, much of the CuAlj may be separated from the 



Menca. Wallenberg.-^ jj^^j^i Treatment of Duralumiu 313 

aluminum by this layer of insoluble FeAlg and is effectually pre- 
vented from dissolving. Thus E3-F, containing only 1.56 per 
cent Cu, heated 20 hours at 500° C and quenched, still contains 
free CuAlj, although its solubility at that temperature was about 
3 per cent. Its structure is shown in Fig. 23. The undissolved 
CuAlj (light) is surrounded by FeAlg (dark). (The other light 
islands are MgjSi, which are distinguishable under the microscope 
as of bluish color, but photograph light.) 




Aluminum 

Fig. 26. — Suggested form of liquidus surfaces of ternary system aluminum-iron-copper 

near aluininum end 

This inclosure of the compound of one binary eutectic by that 
of another seems to be characteristic of light aluminum alloys. 

Fig. 20 shows an island of CuAlj inclosing one of MgzSi. Such 
a structure explains probably the confusing heating and cooling 
thermal curves often obtained with copper-aluminum-magnesium 
alloys. In Fig. 1 6 was shown several normal heating and cooling 
curves for N34 containing both copper and magnesium. The 
inverse heat effect in the quenched alloy at about 260° C and the 
eutectic arrest at 510° C are both visible. In Fig. 17 are shown 
the heating and cooling curve of N28 containing Cu 4.98 per 



314 Scientific Papers of the Bureau of Sta^idards [Vd. is 

cent and Mg 2.41 per cent. On the up curve the usual 520° C 
arrest is noticed; upon cooling, however, instead of one, three 
arrests are noticed, at 502° C, at 478° C, and at 456° C. This 
cycle will repeat itself indefinitely not only in this allov but in 
others containing copper and magnesium, particularly when of 
rather high copper and magnesium content. 

The structure of Fig. 20 was obtained in N28 after the thermal 
analysis was completed and is characteristic ; practically all of the 
MgjSi is surrotmded completely by CuAlj. Upon cooling, CtiAl2 
separates at the first arrest (500° C), at the second and third 
IMgjSi and possibly some traces of Mg^Al-. These crystallize inside 
of the CUAI2 ; the aluminvun particles of the respective eutectics 
coalesce with the aluminum, grains. Upon reheating this alloy the 
surface of contact between IMgjSi and aluminum is so slight that 
the melting of the eutectic, which should normally occur at the two 
lower cooling arrests, proceeds too slowly to give an arrest, and not 
until the protecting sheath of CuAlj melts as eutectic at the higher 
(520° C) arrest does the MgoSi melt also. 

These thermal arrests obtained around 500° C are related to 
the formation of the various eutectics and do not have anything 
to do with the hardening of duralumin. 

VI. CONCLUSIONS RELATIVE TO THE MANUFACTURE AND 
HEAT TREATMENT OF DURALUMIN 

It has been shown that when duralimiin is rapidly cooled by 
quenching from temperatures between 250 and 520° C, and aged 
thereupon at temperatures from o to 200° C, the hardness and, 
at least at lower aging temperatures, the ductility increase. The 
actual values of hardness and ductUity thus obtained depend upon 
the quenching temperatures ; they increase with that temperature 
up to about 520° C, corresponding to the increase of CuAlj in solid 
solution. At this temperature any free CuAlj melts as a eutectic 
and the material is spoiled; this eutectic temperature therefore 
marks the upper limit of the useful quenching temperature range. 

In order to develop the best mechanical properties by heat 
treatment, a quenching temperatiu-e should be used as near this 
as is possible without nmning risk of burning the metal by the 
melting of this eutectic. In practice it should be possible to 
quench from temperatures between 510 and 515° C. 

The period of time at which sheet material should profitably be 
held at the quenching temperature lies between 10 and 20 minutes. 
Heavier sections such as bars might require more time at this 



Merica.waitenberg.-j ^g^,^ Treatment of Duralumin 315 

temperature, as the structure of such sections would be coarser 
and would require somewhat more time for the complete solution 
of the CUAI2. 

Quenching is best and most conveniently carried out in boiling 
water. The mechanical properties are better after quenching in 
hot than after quenching in cold water, and there is less danger 
of cracking due to cooling stresses. 

The best temperature for subsequent aging depends upon the 
mechanical properties that are desired. For most purposes it 
will be found best to age at 100° C for about 5 to 6 days. The 
greater portion of the hardening effect takes place within this 
period. Such a treatment develops both high strength and 
high ductility. If a material having a higher proportional limit 
but lower ductility is desired, the material may be aged at higher 
temperatures up to 1 50° C for from 2 to 4 days. 

The authors' experience has not led them to recommend a 
different composition for duralumin than that in current use; 
that is, Cu, 3 to 4.5 per cent; Mg, 0.4 to i.o per cent; Mn, o to 0.7 
per cent ; 99 per cent Al (remainder) . 

It is believed that it would be of advantage to preheat the 
ingots for hot rolling to a somewhat higher temperature than is 
sometimes used. It would be desirable to preheat to 500° C or 
as near to that temperature as the temperature tmiformity of the 
furnace permitted; the free CuAlj would have better opportunity 
of going into solution at this temperature than at lower ones. 
Rolling, however, can not be done at this temperature, due to 
the eutectic of the MgjSi melting at 450° C and consequent 
hot shortness of the material. It might therefore be advisable to 
preheat to 500° C, but to roll at about 450° C. 

VII. SUMMARY AND CONCLUSIONS 

The heat treatment of alloys of the type, duralumin, was in- 
vestigated and the effect observed of variations in the heat- 
treating conditions, such as quenching temperature, temperature 
of quenching bath, and of aging or tempering, and time of aging 
upon the mechanical properties. 

Conclusions are drawn relative to the best conditions for com- 
mercial heat-treating practice for this alloy. The temperature of 
quenching should not be above that of the CuAlg aluminum 
eutectic, which is usually al)Out 520° C, but should be as near to 
this as possible without danger of eutectic melting. The pieces 
should be held at this temperature from 10 to 20 minutes and 



3i6 Scientific Papers of the Bureau of Standards ivoi. is 

quenched preferably in boiling water. The hardening may for 
most purposes best be produced by aging for about 5 days at 
100° C. 

A theory of the mechanism of hardening of duralumin during 
aging, after quenching from higher temperatures, was developed 
which is based upon the decreasing solubility of the compound 
CuAlj in solid solution in aluminum with decreasing temperatures 
from 520° C to ordinary temperatures. It is believed that the 
precipitation of excess CuAlj which is suppressed by quenching 
proceeds during aging, the precipitation taking place in very 
highly dispersed form. The hardening is due to the formation of 
this highly dispersed precipitate. 

According to this theory the hardening of duralumin during 
aging or tempering after quenching presents a very close analogy 
with that of steel, and the evidence in support of the theory is of 
the same nature and of approximately the same competence as 
that in support of the prevailing theory of the hardening of steel. 

Washington, February 27, 191 9. 



